Chargeless method of producing image on photoconductive surface

ABSTRACT

AN IMAGE DEVELOPING TECHNIQUE ACCORDING TO WHICH AN IMAGE IS DEVELOPED UNDER DARKROOM CONDITIONS FIRST WITH A MEDIUM WHICH IS ADSORBED OR CHEMISORBED BY THE SURFACE AND WHICH EXTENDS THE IMAGE FORMING FIELD BY TRANSMITTING THE CHARGES FROM THE SURFACE TO A HIGHER LEVEL ABOVE THE SURFACE, AND THEN DEVELOPING THE RAISED IMAGE BY A DEVELOPER ATTRACTED TO THE FIELD, THE FIELD EXTENDER BEING CYCLOHEXANE, NITROBENZENE, FURAN, TOLUOL, ACETONE, KETONES, ALDEHYDES, ACIDS AND ESTERS.

Patented Sept. 4, 1973 Int. Cl. 003 13/10, 13/22 US. Cl. 96--1 LY 6Claims ABSTRACT OF THE DISCLOSURE An image developing techniqueaccording to which an image is developed under darkroom conditions firstwith a medium which is adsorbed or chemisorbed by the surface and whichextends the image forming field by transmitting the charges from thesurface to a higher level above the surface, and then developing theraised image by a developer attracted to the field, the field extenderbeing cyclohexane, cycloheXanone, nitrobenzene, furan, toluol, acetone,ketones, aldehydes, acids and esters.

This application is a continuation-in-part of Ser. No. 870,079, filedApr. 17, 1969, which in turn is a continuation-in-part of Ser. No.449,882, filed Apr. 21, 1965, both earlier applications having beenabandoned.

This invention relates to electrostatic photography and radiography andto the production of images on photoconductive surfaces, phosphorsurfaces, metal and other surfaces by the impingement of light or X-rayson said surfaces. In particular this invention relates to a method ofand means for producing images on such surfaces wherein no corona orelectrically produced charge is applied to the surface before exposure,but also applies to charge methods particularly to low charge methods.For convenience images produced on a surface to which there has not beenapplied any corona or electrically produced charge will be termed hereinas chargeless or light-induced images.

In prior art processes it has been claimed to produce images onphotoconductor surfaces by producing in said photoconductive surfaces achange in conductivity wherein the conductivity of the layer isincreased in the lightstruck areas following which the surface ischarged by one component of the dry developer which is applied to thesurface subsequent to the exposure. It is also known to produce on asurface a light-induced charge in the light-struck areas and to developa visible image by means of sensitive liquid developers characterized inthat the developers contain very fine particles of pigment and the likedispersed in an insulating liquid carrier. It is desirable to improvethe density of chargeless images to match the charge methods, and toensure that the polarity of the chargeless image is such as to give thedesired attraction or repulsion development. Therefore it is one objectof the present invention to increase the density of the image, andparticularly though not only chargeless images, and to produce the imagein a better manner. It is another object of the present invention toenable positive reproductions to be made by a process which in itspreferred form involves no corona or electrical charging means toproduce the surface charge.

The chargeless electrophotographic process as at pres ent knowncomprises the steps of exposing a dark-adapted electrophotographicsurface or metal or phosphor surface or metallic salt or organic surfaceto a light or X-ray image and then developing the latent image producedby the impingement of the light or X-ray with developer particles, suchas a pigment, dye, resin, oil or wax which are sensitive to the forcesexerted by a low intensity electric field associated with thelight-induced charge on the surface. Generally the density of the imagesis considerably less than that attained in the charge processes.Improvements have been made in the developed image and in the contrastof the images by the use of background cleaners which reduce backgroundcoloration but a disadvantage of present methods is the difiiculty ofobtaining densities as high as those commonly associated with the chargemethods.

According to the present invention there is provided a method ofproducing and developing an electrical image on a surface whichcomprises first allowing localized charges to be dissipated and whilemaintaining darkroom conditions exposing the surface to patternedradiation to produce image-forming fields thereon in accordance withsaid patterned radiation, and while still maintaining darkroomconditions developing the image-forming fields by applying particulatemarking means from an electrically insulating carrier liquid,characterized by the added step of maintaining on the surface whileeffecting development a thin layer of a material which is absorbed,adsorbed or chemisorbed by the surface and which extends theimageforming fields by transmitting the charges from the surface to ahigher level above the surface.

According to another aspect of the invention, there is provided a memberfor use as a support for an electrical image which is formed by exposinga surface of the member to patterned radiation to produce image-formingfields thereon, and developing the image-forming fields by applyingparticulate marking means from an electrically insulating carrierliquid, characterized by a layer of from 0.1 to 1 micron thick on thesurface of a material which is absorbed, adsorbed or chemisorbed by thesurface and Which extends the image-forming fields by transmitting thecharges from the surface to a higher level above the surface.

The charge methods include the further step of charging an insulator ona surface and bleeding away the unwanted charge by a photoconductorthrough light or X-rays or other electromagnetic means, but even withthe heavier charges then available the invention still improvesdevelopment.

In one method of the present invention the following steps are used toattain the desired objects:

(1) The electrophotographic or metal surface or phosphor surface isdark-adapted by storing said surface in the dark for a period of about24 hours or by subjecting said surface to a temperature of about F. for1 hour and then exposing said surface to the activating radiation whichis in question, whereby the radiation produces in and on the surface anelectrostatic latent image or a change of state which results in alocalization of charge or to produce an effect of polarization such asthe Dember effect. The change of state produced varies in stateaccording to the amount of light which has impinged on it and the resultis one known as a continuous tone latent image.

(2) The surface is then treated with a field-extending material undersafelight conditions whereby a thin film of said material is formed onor attached to said surface and the film or layer of sensitizer isactivated according to the intensity of light which has previouslyimpinged on the surface.

(3) The latent image the field of which is now made more readilyavailable by the action of the absorbed or adsorbed or chemisorbedsensitizer is then developed by the application of an attractable liquiddeveloper which would not normally develop a chargeless image but woulddevelop a charged image of the prior art electrostatic printingprocesses.

In another method of the present invention the surface of theelectrophotographic sheet or phosphor or metal sheet is first treatedwith the field-extending material before exposure of the dark-adaptedsurface. It should be appreciated that the field-extending material isnot in the nature of a chemical photographic sensitizer but is anabsorbed, adsorbed or chemisorbed film or layer of monomolecular ormicellar layer which has the characteristic of making the surface chargemore readily available by transmitting it to a higher level above thesurface.

The field-extending material may comprise a liquid, a solution, a solid,a gas or an emulsion. The materials used cover a wide range of polar andnon-polar substances with a wide range of values of dielectric constantand bulk specific resistivity. Optimum operation of the process dependson the formation of a thin layer of the fieldextending material on theelectrophotographic surface in question, the layer being for examplefrom 0.1 to 1 micron thick. This layer is applied by the followingmeans:

(1) Depositing the layer from a solution by evaporating the solvent, forexample by forced evaporation by air blast or by natural means underambient conditions.

(2) Applying the material to the surface in a liquid and removing theexcess material from the surface with a roller or other squeegee devicewhich squeezes the excess away to leave a film, in which casedevelopment will occur whilst the film is still wet.

(3) Deposition of the required amount of field-extending material on thesurface as a residue following evaporation of a carrier vehicle orsolvent.

This invention does not depend on the use of sensitive fine-particleliquid developers as used in normal chargeless processes which we haveheretofore described but functions successfully with developers designedfor the charge processes.

It is believed that the developer particles which are not ordinarilyretained by the chargeless image are attached to the surface byextending the field effect associated with the image by polarisation ofthe sensitizing material. The force retaining the deposited developerparticles on the image forming surfaces increases as the distancebetween the particles and the attracting charged surface decreases. Theminimum distance is limited particularly by developer particle size asdemonstrated by the inability of the charge developers to retainmaterial on a chargeless latent image. It is apparent that the thinlayer of field-extending material serves virtually to decrease thedistance between the developer particles and the developable surfacecharge by interposing dipoles to span the gap and consequently toprovide a greater attracting force, or otherwise stated to provide anattractive bridge. Alternatively the action of the field-extendingmaterial is to desorb the gaseous barrier provided by the air or otherinsulating barrier and to provide a bridge of higher dielectricconstant. Orientation of the field-extending material is possibly thegoverning factor, be this achieved by polar particles or by insulatedparticles which are polarized by the field to provide the dipole bridgesat images areas, non-orientation at nonimage areas ensuring that nobridges are available and consequently a clean background results.

The field extending material may be applied to the photoconductorsurface of electrophotographic paper or the like during manufacture andthen forms a thin layer in which the polarising or dipole action takesplace under an image field.

In further explanation of this invention it should perhaps be pointedout that according to the normal earlier used methods of producing anddeveloping xerographic images it was necessary for the photoconductivesurface to have an insulating layer or characteristic such that a chargecould be applied to the surface by corona discharge means or the likewhich is held on the surface as long as dark conditions are maintained,the photoconductor however bleeding away this charge from the surfacewhen it is struck by light so that, after charging, such a surface canthen have a latent electrostatic image formed thereon by projecting alight image on to the surface, this latent electrostatic image beingmaintained so long as the image-bearing member is maintained under darkconditions. The image is rendered visible by applying particles of tonerto the surface which will be either attracted to the image areas or insome cases will be forced to the non-image areas.

It is customary to provide means for fixing the image such as byincluding a fixing substance in a carrier liquid which will bond theimage to the surface on evaporation of the carrier liquid or accordingto older methods it was customary to heat-fuse the image to the area.

At that stage it was generally believed that it was necessary to have acharge on the surface which could be bled away differentially, but morerecently it has been shown that a charge can be produced on a surface bysimply subjecting it to electromagnetic waves such as light waves orX-rays or the like, which charge, though very much less than the latentelectrostatic image produced by the older methods, is neverthelessdevelopable by a sensitive developer, but naturally as a lower orderforce exists on the electrostatic latent image, the densities attainableare not as good as with the methods where a charge is applied to theinsulating surface and is then modified by the light image.

It must be clear at this stage therefore that according to the methodsgenerally adopted heretofore a much heavier charge existed because thecharge was applied to the insulative surface and then bled away wherenot required whereas in the later chargeless methods no charge whateveris applied and the effect is simply one of modifying the surface orinterface materials themselves to give an image which can be developedelectrically.

It is found that no insulating surface in the normal sense is necessarybecause the light-induced charges appear to be associated with themolecular structures of the material itself and for this reason thepresent invention applies to oxidizable or non-oxidizable surfaces.

Where there is no applied charge, it will be realised that the imagewhich can be developed would be of a lower order in magnitude, andtherefore the present invention is highly important in this field as itallows conditions to obtain which aid the development by providingbridging means or concentrating means whereby the relatively low orderfield can still give an image corresponding approximately to the imagewihch can be obtained where charge methods are involved, and chargemethods can naturally give better densities than previously.

It should be clear therefore that the invention is particularly suitablefor chargeless development but it also applies to any method involvingdevelopment of an electromagnetically induced image.

To illustrate the manner in which the invention is carried out thefollowing examples are given relating to three modifications:

EXAMPLE 1 In this example the surface which was used to produce thelatent chargeless image comprises a layer of photoconductive particlesbound by an organic resin or other binder and coated on a backing suchas paper, metal or plastic sheet. The sheet was dark-adapted by leavingthe sheet in the dark for a period of 24 hours following which the sheetwas removed to an exposing position where a contact print or aprojection print was made in the following manner. The sheet was exposedto a light source for 5 seconds using a blue actinic watt lamp at adistance of 2 feet from the upper surface. After the exposure, the paperbacked sheet was placed in a dish in which it was subjected to theaction of a field-extending liquid for a period of 5 seconds, thefield-extending liquid consisting of a technical grade of heptane, anonpolar hydrocarbon liquid. After removal from the dish the Wet surfacewas allowed to dry in the atmosphere to the point where a change in thereflectance of the surface occurred giving a matt appearance indicatingthat the critical amount of material had been retained in an absorbed oradsorbed form. The sheet was then immersed in a developer, of which theparticles were positively charged, contained in a dish. The compositionof this developer has been described in prior patents of the presentinventors wherein carbon black, reflux blue, and resins such as long oilalkyd resins are used to provide control and fixing actions. Theconcentration of the developer was limited to approximately 14% solidsin the insulating carrier liquid. A development time of 5- seconds wasrequired to produce a dense image. Upon removal from the developing baththe excess developer was removed by subjecting the surface to cleancarrier liquid.

EXAMPLE 2 In the process of Example 1, the following liquid wassubstituted for the heptane: cyclohexane, commercial grade. Thefollowing liquids may also be employed: benzene, carbon tetrachloride,and carbon disulphide, other hydrocarbons and halogenated hydrocarbons.

EXAMPLE 3 In the process of Example 1, the following liquids of polartype may be substituted for the heptane: cyclohexanone, nitrobenzene,furan, toluol, and acetone, and other ketones, aldehydes, acids andesters.

EXAMPLE 4 In this embodiment of the present invention the expo sure stepand the developing method were similar to those of Example 1 but theheptane was replaced with a mixture or solution of non-polar paraffinoil in heptane in the proportions 2 to percent of oil in the heptane,the heptane still constituting the field-extending material.

EXAMPLE 5 In the process of Example 4, the following liquids of lowevaporation rate may be substituted for the paraffin oil added to theheptane; sunflower oil or linseed oil or animal fats, silicone oil, longoil or alkyd resin.

EXAMPLE 6 EXAMPLE 7 In this modification of the present invention, theelectrophotographic sheet of Example 1 was replaced by metal sheet suchas silver, lead or zinc.

EXAMPLE 8 In this modification of the present invention, theelectrophotographic sheet of Example 1 was replaced by a selenium plate.

What we claim is:

117-37 LE; 250- R 1. A chargeless method of producing and developing anelectrical image on a photoconducor surface which consists essentiallyof first allowing localized charges to be dissipated and whilemaintaining darkroom conditions exposing the photoconductor surface topatterned radiation to produce image-forming electrical fields thereonin accordance with said paterned radiation, and while still maintainingdarkroom conditions developing the imageforming fields by applyingparticulate marking means from an electrically insulating carrierliquid, and maintaining on the surface while effecting development athin layer of between 0.1 and 1 micron of a polar substance orientatableby the image forming fields, which substance is applied from a liquidand at least partially dried to be absorbed, adsorbed or chemisorbed bythe surface and which extends the image-forming fields by transmittingthe charges from the surface to a higher level above the surface, saidsubstance being cyclohexane, cyclohexanone, nitrobenzene, furan, toluol,or acetone.

2. A method according to claim 1, wherein the fieldextending material isapplied before imagewise exposure.

3. A method according to claim 1, wherein the fieldextending material isapplied after imagewise exposure but before development.

4. A chargeless method of producing and developing an electrical imageon a photoconductor surface which consists essentially of first allowinglocalized charges to be dissipated and while maintaining darkroomconditions exposing the photoconductor surface to patterned radiation toproduce image-forming electrical fields thereon in accordance with saidpatterned radiation, and while still maintaining darkroom conditionsdeveloping the imageforming fields by applying particulate marking meansfrom an electrically insulating carrier liquid, and maintaining on thesurface while effecting development a thin layer of between 0.1 and 1micron of a polarizable substance orientatable by the image formingfields, which substance is applied from a liquid and at least partiallydried to be absorbed, adsorbed or chemisorbed by the surface and whichextends the image-forming fields by transmitting the charges from thesurface to a higher level above the surface, said substance beingnon-polar paraflin oil in heptane or a vegetable oil or animal fats,silicone oil or an alkyd resin or carbon tetrab-romide in a chlorinatedfluorinated hydrocarbon liquid.

5. A method according to claim 4 wherein the fieldextending material isapplied before imagewise exposure.

6. A method according to claim 4 wherein the fieldextending material isapplied after imagewise exposure but before development.

References Cited UNITED STATES PATENTS 2,990,280 6/1961 Giaimo 96-13,068,115 12/1962 Gundlach 117-37 3,247,007 4/1966 Oliphant 117--373,251,688 5/1966 Mihajlon 96--1 3,032,432 5/1962 Metcalfe et a1. 11737GEORGE F. LESMES, Primary Examiner M. B. WITTENBERG, Assistant ExaminerUS. Cl. X.R.

